1. Technical Field
Embodiments of the present disclosure are related generally to bent-axis hydraulic machines, and in particular to machines in which unswept cylinder volume is controlled to reduce efficiency losses that arise because of compression of hydraulic fluids during operation of the machines.
2. Description of the Related Art
Hydraulic machines are in common use in a wide variety of industrial, commercial, and consumer applications. Hydraulic machines transmit power by conducting pressurized fluid between low pressure and high pressure reservoirs. One general category of hydraulic machines includes machines that employ a rotating barrel with a plurality of pistons positioned in respective cylinders formed in the barrel, each lying parallel to a common axis, and can be called axial piston machines. This general category can in turn be divided into at least two major classes: swash plate and bent-axis.
In both classes, fluid pressure in the cylinders drives the pistons against a plate that lies at an angle with respect to the barrel. In swash plate machines, the barrel rotates on a common axis with a mechanical power shaft of the machine, while the plate is positioned at an angle to both the barrel and the shaft, and does not rotate. In bent-axis machines, the barrel is placed at an angle with respect to the shaft, while the plate lies perpendicular to the shaft and rotates with the shaft. The angle of the barrel (or swash plate, in the case of that class of machine), relative to the shaft, is variable, to vary the displacement of the machine.
Generally speaking, the most efficient and versatile of these machines are the bent-axis machines, which are frequently used for power applications in heavy equipment such as construction and earth moving machines, and may be used to power hybrid vehicles.
The basic design of most axial piston machines potentially allows them to operate both as fluid pumps and as fluid motors, and so these devices are often referred to as pump/motors. When acting as a pump, mechanical power from an external source acts on the mechanical power shaft, which acts as an input shaft to drive the piston/cylinder assembly in a way that creates reciprocal motion of the pistons that in turn results in the pumping of fluid from a low pressure fluid reservoir to a high pressure reservoir. When acting as a motor, fluid from the high pressure reservoir flows in a reverse manner through the piston/cylinder assembly to the low pressure reservoir, causing a reciprocal motion of the pistons that now delivers mechanical power to the shaft, which now acts as an output shaft.
The operation of a typical bent-axis pump/motor 100, operating as a motor, will be described in more detail with reference to FIGS. 1A-1C. While its operation as a pump will not be described in detail, both operations, as a motor and as a pump, are well known in the art.
The term axial force is used herein to refer to force vectors that lie substantially parallel to a defined axis, while the term radial force is used to refer to force vectors that lie in a plane that is substantially perpendicular to a defined axis. Neither term is limited to vectors that intersect the axis. In particular, the radial forces referred to herein generally lie in vectors some distance from the defined axis such that a device that is configured to rotate about the axis, and upon which the radial forces act, will tend to rotate in reaction to the forces.
FIGS. 1A-1C show views of selected elements of the bent-axis pump/motor 100 (referred to hereafter as motor) according to known art. The motor 100 includes a back plate 102 and a cylindrical barrel 104, having a plurality of cylinders 106 within which pistons 108 travel reciprocally. The pistons 108 each have a sliding seal engagement with the wall of the respective cylinder 106, at respective first ends, and engage a socket formed in a drive plate 110, at respective second ends 109 in a ball-and-socket-type coupling. The drive plate 110 is coupled to a mechanical output shaft 120 that is rotationally driven by the motor 100. Typically, bent-axis pump/motors are provided with an odd number of cylinder/piston pairs, usually seven or nine. Cylinder barrel 104 is shown in cross section so that two of a plurality of cylinder/piston pairs are directly opposite each other, as would occur in a barrel having an even number of cylinder/piston pairs, in order to more clearly illustrate the top and bottom limits of travel of pistons 108 within respective cylinders of the cylinder barrel 104, and the relative volumes of fluid constrained by the pistons 108 at the top and bottom of rotation. However, the principles described are common to most bent-axis machines, whether having an even or an odd number of cylinder/piston pairs. Cylinder 106A and piston 108A are shown positioned at the top of the barrel 104 as viewed in the drawings, while cylinder 106B and piston 108B are shown at the bottom of the barrel.
The cylinder barrel 104 is configured to rotate around a first axis A with a face of the cylinder barrel 104 slideably coupled to a valve face 113 of the back plate 102, which does not rotate. While many designs provide a back plate and a valve plate as separate elements, for the purposes of the present disclosure, they will be shown hereafter as a single integrated component. The drive plate 110 rotates around a second axis B at a common rate with the cylinder barrel 104. Typically, a universal joint (not shown) couples the barrel 104 to the drive plate 110.
As the cylinder barrel 104 rotates around axis A, each cylinder 106 follows a circular path around axis A. Because the drive plate 110 rotates at the same rate around axis B, and because second ends 109 of pistons 108 are engaged with drive plate 110, first ends of pistons 108 are caused to reciprocate within respective cylinders 106 except when the axes A and B are coaxial, as shown in FIG. 1C. The uppermost point of that circular path, as viewed in the drawings, is referred to herein as top-dead-center, indicated in FIGS. 1A-1C as TDC, while the lowermost point is referred to as bottom-dead-center, indicated in FIGS. 1A-1C as BDC.
The cylinder barrel 104 and back plate 102, which define axis A, are configured to pivot around a third axis C, with respect to the drive plate 110 and shaft 120, which define axis B, for the purpose of varying the displacement volume of the pump/motor 100, as explained below. Axes A and B lie in the plane of the drawings, while axis C extends normal to the plane of the drawings, and so appears as a point in FIGS. 1A-1C. The degree to which axis A pivots away from a coaxial relationship with axis B is referred to herein as the stroke angle of the motor. The stroke angle determines the distance each piston 108 travels within its respective cylinder 106 as the cylinder barrel 104 rotates, and the volume of the cylinder swept by the piston per rotation of the barrel, and thereby determines the amount of fluid displaced in each revolution, also known as the displacement of the motor. When axes A and B are coaxial, the stroke angle is generally referred to as being at a zero angle, or at a minimum angle, and the pistons do not reciprocate. The stroke angle can be described in terms of degrees—i.e., the angle of axis A relative to axis B—or of a percentage of the maximum angle possible for a given machine, or in more general terms, such as small, large, maximum, minimum, etc.
It is common in the art to establish the stroke angle by providing a pivoting structure known as a yoke, which carries the back plate and cylinder barrel, and pivots around axis C to establish the stroke angle. A yoke commonly includes one or two legs that pivot about respective trunnions rotatably supported by a casing of the motor. It is also common for one or both yoke legs to incorporate fluid passages that provide for a flow of pressurized fluid between the back plate and the trunnions. As used herein, the term yoke refers to a structure having one or two legs that pivot at a first end about a trunnion and carry a back plate and cylinder barrel at a second end through a stroke angle. A yoke may include an integral portion that constitutes a back plate, or have a distinct back plate structure thereto attached.
FIG. 1C shows, in dashed lines 102a, 102b, the relative position of the back plate 102 at the stroke angles shown in FIGS. 1A and 1B, respectively. It can be seen that an arbitrary reference point P on the back plate 102 follows an arc E1 that is centered on axis C as the back plate 102 changes stroke angle.
FIG. 1A shows motor 100 at a maximum, or 100% stroke angle, which results in a maximum displacement of the motor 100 and a maximum degree of energy transfer. FIG. 1B shows the motor 100 positioned at a moderate stroke angle of approximately 50%, and FIG. 1C shows the motor 100 at a stroke angle of zero, in which the axes A and B are coaxial, and energy transfer is virtually zero.
The term displacement is used to refer to the total volume in the cylinders 106 that is swept by the pistons 108 during a single rotation of the barrel 104. Displacement includes a numerical value and a unit indicating a volumetric measure, such as cubic centimeters, etc. This volume is the amount of fluid that will pass through the motor during each revolution of the shaft 120. Given the displacement value of a pump and its rate of rotation, it can easily be determined how much fluid will be moved over time. When employed as a motor, the displacement value of the machine defines, in conjunction with other pertinent measures such as fluid pressure, the output torque of the machine at that displacement.
In each of the FIGS. 1A-1C, the piston 108a positioned in cylinder 106a at TDC lies at the outermost limit (OL) to which it will travel within the cylinder over the course of a rotation of the barrel 104, given the stroke angle shown. The position of the face of the piston 108a is indicated at line OL. Similarly, piston 108b, positioned in cylinder 106b at BDC, lies at the innermost limit (IL) to which it will travel within the cylinder 106b over the course of a rotation of the barrel 104. The position of the face of the piston 108b is indicated at line IL. In any given cylinder 106, the volume that lies between the lines OL and IL represents the displacement of that cylinder 106 at that particular stroke angle. Thus, the displacement of the pump/motor 100 is the sum of the displacements of all of the cylinders 106 of the device at that stroke angle.
When the motor is at its maximum stroke angle, as shown in FIG. 1A, the lines OL and IL lie a maximum distance apart. This is the maximum displacement per cylinder that can be achieved by the pump/motor 100, and provides the highest degree of energy transfer from the high-pressure fluid to the rotation of the drive plate 110, in the form of torque. FIG. 1B shows a moderate stroke angle of about half the maximum angle. It can be seen that the lines OL and IL lie closer together than in FIG. 1A. At this smaller angle, a smaller degree of energy transfer is achieved. When the pump/motor is at a zero stroke angle, as shown in FIG. 1C, the lines OL and IL define the same point because, even though the barrel 104 rotates while at this stroke angle, the pistons 108 do not move axially within the respective cylinders 106, and so do not sweep any volume, but remain substantially stationary near a mid-point of the respective cylinder 106. At the stroke angle shown in FIG. 1C, the drive motor 100 is at zero displacement, and does not exert any radial force to the output shaft 120.
The valve face 113 has two semicircular fluid ports over which the cylinder barrel rotates, so that each cylinder 106 is in fluid communication, first with one of the fluid ports for about half of each rotation, and then with the other of the ports for the other half rotation. One fluid port is coupled to a high-pressure fluid supply, and the other to a low-pressure supply. When the pump/motor 100 is operating in a motor mode, high-pressure fluid begins to enter each cylinder 106 as the respective cylinder passes TDC, and continues to enter until the cylinder reaches BDC. The high-pressure fluid applies a driving force on the face of the respective piston 108 that acts on the piston axially with respect to axis A. This force is transferred by the piston 108 to the drive plate 110 as the barrel 104 rotates through 180 degrees, until the respective cylinder 106 passes BDC, at which point the cylinder is placed in fluid connection with the low-pressure fluid supply, and the piston 108 pushes the fluid out of the cylinder 106 as the cylinder 106 continues to rotate back toward TDC.
Referring to FIG. 1A, it can be seen that the driving force on the pistons 108 is axial, relative to axis A, but will include both axial and radial force components, relative to axis B. The distribution of the driving force between the axial and radial components depends on the stroke angle of the pump/motor, and can be calculated in accordance with well known and long established mechanical principles. The axial component will tend to exert a force on drive plate 110 away from the barrel 104 along axis B, which is resisted by elements such as thrust bearings etc., which are well known in the art. The radial component will exert a force on the socket of the drive plate 110, into which the second end 109 of a given piston 108 is seated to urge that socket downward, as viewed in the drawings, causing drive plate 110 to rotate so that that socket moves further away from the barrel 104, with the barrel 104 rotating in unison with the drive plate. Only radial force is converted to energy, in the form of torque.
The smaller the stroke angle, the more of the exerted force will be distributed to the drive plate as an axial force, until, at a zero stroke angle, such as that shown in FIG. 1C, all of the exerted force is distributed to the drive plate 110 as an axial force, with none being applied as a radial force. Accordingly, even though the cylinders 106 are fully pressurized at the zero stroke angle, there is no torque applied to the output shaft 120, which is therefore free to rotate independently of the fluid system. In the case of a vehicle that is powered by such a motor, a zero stroke angle might be indicated when the vehicle coasts, and no power needs to be delivered to or retrieved from the drive wheels.
Hydraulic bent-axis pump/motors are described in a large number of patents, including the following U.S. Pat. Nos. 3,760,692; 4,034,650; 4,579,043; 5,488,894; 5,495,912; 6,257,119; 6,874,994; and 7,594,802, all of which are incorporated herein by reference, in their entireties.
In FIGS. 1B and 1C, it can be seen that a volume 119 exists beyond the outer limit of travel of the piston 108a at TDC that is not swept by the piston at that stroke angle. Owing to the geometry of the device, a significant unswept volume will exist at any stroke angle smaller than the maximum angle (shown in FIG. 1A), with the largest unswept volume occurring at a zero stroke angle. As the device operates, this unswept volume 119 is always occupied by fluid, which is subjected to low and high pressure extremes during the course of each revolution even though it does not participate in transmitting power.
While hydraulic fluids are considered to be effectively non-compressible in many contexts, they are in fact slightly compressible, leading to undesirable mechanical effects such as fluid hammer, noise, and volumetric leakage. The unswept volumes 119 of the depicted prior art bent-axis design are a source of such undesirable effects. Because the potential for such effects tends to be proportional to the volume compressed and the magnitude of pressure, prior art bent-axis machines are particularly susceptible to these effects at high operating pressures and virtually all stroke angles. This poses a problem for their use in hybrid vehicle applications, because such applications tend to call for high maximum operating pressures, and high efficiency and minimum noise over a broad range of stroke angles.
Compressibility-related leakage is a particular concern for efficiency. Within the range of fluid pressures that are typical with hydraulic motors, the volumetric compressibility of hydraulic fluid is generally around 1% per 1,000 psi. Thus, if the high-pressure fluid supply of a motor is at 5,000 psi, the fluid in each of the cylinders will compress by about 5% each time the respective cylinder switches from low pressure to high pressure, and decompress by the same amount each time the respective cylinder switches from high pressure to low pressure. This means that whatever the stroke angle of the motor, an amount of fluid equal in volume to about 5% of the fluid in the cylinder will be lost to the low-pressure side of the system each time the cylinder crosses BDC.
Referring again to FIGS. 1A-1C, the volume of fluid in the cylinder 106b at BDC (where it switches to low pressure) decreases as the stroke angle diminishes, which means that the fluid loss due to fluid compressibility also diminishes. However, the impact of that fluid loss on motor efficiency increases as stroke angle diminishes, because the fluid loss—which is directly proportionate to the fluid volume at BDC—drops by about 50% between maximum displacement and zero displacement, while power output of the motor drops by 100% over the same range. It can be seen, for example, that at a minimum stroke angle, as shown in FIG. 1C, where the pistons 108 do not move axially in respective cylinders 106 as the cylinder barrel 104 rotates, each cylinder is continually about half full of fluid. Each time the cylinder crosses TDC, a small amount of fluid is added as this volume of fluid is compressed by about 5% (assuming a fluid pressure of around 5,000 psi). As each cylinder subsequently crosses BDC, the fluid decompresses and the small amount of fluid escapes to the low-pressure side. Thus, when the motor is coasting at zero displacement, there is no power output, but the fluid loss is still about half of the maximum. In motors that frequently operate in the lower displacement range, this can have a significant impact on overall efficiency of the motor.
Noise and vibration are another concern. As the fluid in each cylinder is compressed at TDC, and again as it is decompressed at BDC, a small pulse, or fluid hammer, is generated. If there are nine cylinders in the cylinder barrel, eighteen such pulses will be generated for each revolution of the barrel. These pulses create vibration and noise in the motor as it rotates. Such vibration has not previously been a particular concern because use of hydraulic machines of the kind described above has traditionally been substantially limited to applications in heavy industries and the industrial workplace. However, as hydraulic machines are being adapted for use in hybrid vehicles that operate on public roads and carry passengers, noise and vibration become a much more important consideration, affecting the comfort of people around the vehicles as well as that of the passengers. Passenger vehicles, especially, are subject to highly competitive consumer marketing, and undesirable noise or vibration can have a significant negative impact on the market value of a vehicle.
The problem posed by unswept volume remaining in the cylinders at small stroke angles has been addressed to some degree in prior art. For example, U.S. Pat. No. 3,760,692 (Molly) discloses a bent-axis hydrostatic drive unit that utilizes an off-center pivot such that the dead space within each cylinder is reduced at all stroke angles. The off-center pivot serves to vary the axial distance between a drive plate and a cylinder barrel as a function of stroke angle, in order to modify the outer limit of reciprocation of the pistons to a point closer to the outer end of the cylinders. Bent-axis hydraulic machines that seek to modify unswept volume as a function of stroke angle in this way will henceforth in this disclosure be referred to as variable length. In this usage, variable length refers specifically to the variable nature of the axial distance between a drive plate and a cylinder barrel as a function of stroke angle.
Advantages of a variable length design will be made more apparent with reference to FIGS. 2A-2C. A hydraulic motor 200 is shown in FIG. 2A at a similar stroke angle to that of motor 100 as shown in FIG. 1A, and, likewise, the motor 200 of FIGS. 2B and 2C is shown at angles that correspond to the angles of the motor 100 as shown in FIGS. 1B and 1C, respectively.
The cylinder barrel 104 and back plate 202 of motor 200 can be seen to move closer to drive plate 110 as they pivot together about axis C from a larger stroke angle to a smaller stroke angle. The result of this movement is that the outer limit of travel of the pistons 108 at TDC remains close to the outermost end of respective cylinders 106, regardless of the stroke angle. For example, referring to FIG. 2C, it can be seen that pistons 208a and 208b are both positioned at the outer end of the cylinder 106, and unswept volumes 119a, 119b between the outer limits OL and the outer ends of the cylinders 106 is virtually zero.
The dashed lines 202a and 202b of FIG. 2C show the relative positions of the back plate 202 at the stroke angles shown in FIGS. 2A and 2B, respectively. In contrast to the prior art motor 100 of FIGS. 1A-1C, a reference point P on the back plate 202 follows an arc E2 that is not centered on axis C, but that is centered on an axis F near the second end 109 of the piston 108a. The amount of compression loss typically associated with smaller stroke angles is thereby reduced, improving the efficiency of the motor 200 and reducing its noise and vibration.
However, the design of FIGS. 2A-2C, using an off-center pivot around which the back plate and cylinder barrel pivot, inhibits over-center operation because one side of the pivoting structure is rigidly fixed. Also, the maximum angle of pivot is potentially limited by geometric interference such as piston rod contact with the cylinder barrel.